1. Field of the Invention
The present invention relates to a novel fluorine-containing compound useful as a lubricant for precision machines and precision parts which require high-precision lubrication, or as a surfactant, a mold-release agent, a rust preventive, etc.; a process for producing the fluorine-containing compound; a lubricant composition comprising the fluorine-containing compound; and a magnetic recording medium such as a magnetic tape, a magnetic disk, etc., the lubricant layer of which comprises the fluorine-containing compound.
2. Description of the Prior Art
With minuaturization and an increase in precision of machines and parts thereof, the mode of lubrication in their sliding portions has come to be changed from hydrodynamic lubrication to boundary lubrication. Particularly in electronic machines and electronic parts, for example, VTR and magnetic discs, high-precision lubrication has become necessary for sliding a magnetic tape or a magnetic disc on a magnetic head, because of employment of thin ferromagnetic metal films for improving the packing density. For example, in metallized tapes and hard discs, a lubricant layer on the surface of a magnetic layer is formed to a thickness of only several tens angstroms to increase the output by reducing the loss due to the spacing between a magnetic recording medium and a magnetic head as much as possible while assuring the durability and the practical reliability. Therefore, it is an important problem to develop an organic compound having an excellent lubricating capability, as a material for forming the lubricant layer.
As lubricants for thin-metal-film type magnetic recording medium, those having a fluorocarbon chain in the molecule are excellent in compatibility with thin metal films, and hence there have been proposed various fluorocarbon-based lubricants (for instance, Japanese Patent Application Kokai Nos. 61-107527, 61-107528, 61-107529, 62-92225, 62-92226 and 62-92227). In addition, compounds comprising a perfluralkyl polyether chain have been proposed as lubricants for magnetic recording medium (for instance, U.S. Pat. No. 3,778,308 and Japanese Patent Application Kokai No. 60-109028).
On the other hand, fluorine-containing compounds having the molecular structure shown below have been proposed as surfactants: ##STR2## wherein R.sub.6 is a hydrogen atom or an aliphatic alkyl group having 1 to 12 carbon atoms, R.sub.8 is an aliphatic alkylene group having 1, 2 or 5 carbon atoms, and h is zero or an integer of 1 to 8 (U.S. Pat. No. 3,798,265).
The following is an important requirement which a lubricant for thin-metal-film type magnetic recording medium should satisfy. The lubricant should adhere strongly to the surface of the thin metal film or the protective film of the medium and the surface of a magnetic head to form a coating layer on each of the surfaces, and groups of molecules of the lubricant should easily be sheared from one another on the contact point of the coating layers, namely, on the surface on which the magnetic recording medium and the magnetic head slide.
Perfluoroalkyl polyethers heretofore well-known as lubricants for magnetic recording medium are good in the ease of shearing between groups of molecules because substantially the whole surface of each molecule is covered with fluorine atoms. But since the polarity of the molecules is low, the conventional lubricants have a low adhesive strength to the surface of a thin metal film or a protective film and the surface of a magnetic head. For removing this defect, there have been proposed perfluoroalkyl polyethers having various polar groups introduced into the ends of the molecule. But when their molecular weight is 3,000 or more, no sufficient effect of the introduction of the polar groups can be obtained. On the other hand, when the molecular weight is reduced for enhancing the effect of the polar groups, the interaction between molecules of the perfluoroalkyl polyether is weak, so that the amount of such a lubricant is decreased by its evaporation. Thus, the perfluoroalkyl polyethers and the terminally modified products thereof are poor in adhesive properties to the surface of a thin metal film or a protective film and the surface of a magnetic head, and in stability. Consequently, thin-metal-film type magnetic recording media obtained by using these compounds as lubricants have been poor in reliability on durability, in particular, in performance characteristics in low-humidity circumstances in which the aggregation of a magnetic metal tends to occur on the surface of a magnetic head.
On the other hand, the fluorocarbon-based lubricants disclosed in the above prior art references have heretofore been used in thin-metal-film type magnetic recording media because of their excellent compatibility with thin metal films, but have been disadvantageous in that they cannot give a sufficient durability in a low-humidity circumstance of 10% RH or less.
Conventional processes for producing a fluorine-containing compound are described below.
A process for producing a fluorine-containing alkylsuccinic acid derivative has been proposed in Japanese Patent Application Kokai No. 61-257226. Specifically, in this process, the fluorine-containing alkyl-succinic acid derivative is obtained by subjecting a mixture of equimolar amounts of a higher alcohol having a perfluoroalkyl group and alkylsuccinic anhydride to reaction with heating at 60.degree. to 150.degree. C. in the atmosphere. This process is effective against perfluoroalkyl group, but has been disadvantageous in that when an alcohol having a fluoroalkyl ether group is used in place of the higher alcohol having a perfluoroalkyl group, the yield of the fluorine-containing alkylsuccinic acid derivative is as very low as 10% or less because the reactivity of the former alcohol is considerably lower than that of the latter alcohol. Furthermore, this process has been disadvantageous also in that since the position of attachment of the fluoroalkyl ether end group to an aliphatic-alkylcarboxylic acid is limited to the carbon atom at the .alpha. or .beta. position adjacent to the carboxyl group, there cannot be obtained a fluorine-containing compound in which the fluoroalkyl ether end group is attached to any of various positions from .alpha. position to .omega. position.
A process for producing a nitrogen-containing perfluoro(carboxylic acid fluoride) has been proposed in Japanese Patent Application Kokai No. 64-70450. Specifically, in this process, a perfluoro(3-dialkylamino-n-butyric fluoride) is obtained by electrolyzing a reactive derivative of 3-dialkylamino-n-butyric acid in liquid hydrogen fluoride. This process has been disadvantageous not only in that both of the two aliphatic hydrocarbon end groups in the molecule of the product are perfluoroalkyl groups, but also in that it is difficult to utilize an aliphatic long-chain alkyl group or an aliphatic polyalkyl oxide group.
A process for producing a phthalic acid diester having a perfluorobutenyloxy group has been proposed in Japanese Patent Application Kokai No. 61-24547. Specifically, in this invention, the phthalic acid diester having a perfluorobutenyloxy group is obtained by adding 2 to 20 moles of an aliphatic alcohol having an alkyl group having 1 to 15 carbon atoms and 0.01 to 1 mole of concentrated sulfuric acid per mole of a phthalic anhydride derivative having a perfluorobutenyloxy group, and carrying out the reaction with refluxing the alcohol or at 100.degree. to 150.degree. C. for 30 minutes to 24 hours. This process has been disadvantageous not only in that both reactive groups of the acid anhydride becomes ester linkage groups, but also in that a fluoroalkyl ether group cannot be utilized because a fluoroalkyl group as the end group is limited to perfluorobutenyloxy group.
A process for producing a perfluoroalkyl-substituted half ester or a perfluoroalkyl-substituted half amide has been proposed in Japanese Patent Application Kokai No. 54-84524. Specifically, in this process, the perfluoroalkyl-substituted half ester or half amide is obtained by reacting maleic anhydride with a hydroxy-substituted or amino-substituted nonionic compound (e.g. an aliphatic alkyl alcohol or an aliphatic alkyl amine) at room temperature or at 50.degree. to 100.degree. C. to obtain a maleic acid half ester or a maleic acid half amide, and then adding a perfluoroalkylalkylene thiol to the thus obtained half ester or half amide at 30.degree. to 75.degree. C. in the presence of a base catalyst. The perfluoro-alkyl-substituted half ester or half amide can be obtained also by reversing the order of the above reactions. These processes have been disadvantageous in that the product obtained by them is a mixture of compounds having a thioether linkage group to the carbon atom at the .alpha. or .beta., respectively, position adjacent to the carboxyl group, so that there cannot be obtained a single fluorine-containing compound having a thioether linkage group only to the carbon atom at the .alpha. position.